The idea of DF-21D is this.
Each missile carries warheads and decoys. If multiple DF-21D coming down. Is the carrier group gonna engage and shoot every single decoy and warhead?
SM missile gonna take out all warheads and decoys regardless whether its has good targeting. it cannot take chances. It can overload the system.
Keep in that swarms of antiship missiles coming side ways too and some of them reach Mach 4 speed,
It's a game changer in a sense that before this, carrier group doesn't need to worry about Mach10 warheads and decoys raining on top of its head
Now it has to worry regardless whether that missile has good targeting because you just don't know and can't take chances and therefore has to take them out high up in the atmosphere, This creating alot more strain and stress to combat system.
When you under stress for certain things you are expecting, then most likely you will likely get killed by something else unexpected.
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